This invention relates to dispersion compensation in optical fibre transmission.
Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55 xcexcm (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity multi-wavelength optical networks.
More importantly, most fibre which has already been installed for telecommunication links (ie. standard non-dispersion shifted fibre) exhibits a dispersion zero around 1.3 xcexcm and thus exhibits high (about 17 ps/nm.km (picosecond per nanometer-kilometer)) dispersion around 1.55 xcexcm. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelengths to 1.55 xcexcm where dispersion-compensation becomes a necessity.
Several techniques have been demonstrated including laser pre-chirping (reference 1xe2x80x94below), mid-span spectral-inversion (phase-conjugation) (reference 2xe2x80x94below), the addition of highly-dispersive compensating fibre (reference 3xe2x80x94below) and chirped fibre gratings (references 4 to 7xe2x80x94below). Chirped fibre gratings are of particular interest, since they are compact, low-loss and offer high negative-dispersion of arbitrary and tunable profile. In separate experiments 450 fs (femtosecond) pulses have been successfully reconstructed after transmission through 245 m of fibre (reference 4xe2x80x94below), and gratings with dispersion equivalent to 20 km and 1 km of standard fibre have been fabricated (references 5 and 6xe2x80x94below). Whilst more recently a grating has been employed to compensate the dispersion of 160 km of standard fibre in a 10 Gbitsxe2x88x921 (gigabits per second) externally modulated experiment (reference 7xe2x80x94below) although no information of the grating strength was given in this case.
It is a constant aim to improve dispersion compensation techniques in optical fibre transmission systems.
The article in IEEE Photonics Technology Letters, April 1993, USA, vol. 5, no. 4, pages 425-427, Farre J et al: xe2x80x9cDesign of bidirectional communication systems with optical amplifiersxe2x80x9d, discloses the use of optical amplifiers at various positions in an optical fibre link.
The article in Optics Letters, vol. 19, no. 23, Dec. 1, 1994, Washington US, pages 2027-2029, Lauzon et al: xe2x80x9cImplementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradientxe2x80x9d, discloses (as the title suggests) the manufacture of chirped fibre gratings by imposing a temperature gradient onto a fibre grating.
GB-A-2 161 612 discloses a chirped fibre grating for dispension compensation in an optical fibre link.
This invention provides an optical transmitter for use with an optical fibre transmission link, the transmitter comprising:
a light source capable of direct or indirect modulation; and
an optical amplifier;
characterised by:
a chirped grating to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source.
Preferably the optical amplifier is operable in a saturation mode.
It is advantageous to position the compensating grating at the input end of the link, since in this position the optical input signal is still relatively large and thus a relatively insignificant noise penalty is incurred. In addition, if the grating""s (compensated) output is then routed to an optical amplifier operating in saturation, the amplifier""s output power will be effectively unaltered by the presence of the compensating grating.
The skilled man will appreciate that the dispersion compensation in this context need not be complete, but simply that the non-linear response of the grating acts against the dispersion characteristics of the transmission link.
This invention also provides an optical fibre transmission system comprising:
an optical fibre transmission link; and
an optical amplifier disposed at an input end of the link;
characterised by:
a chirped grating disposed at the input end of the link, the chirped grating providing compensation against the dispersion characteristics of the link.